1. Field of the Invention
The present invention relates to high pressure steam turbines, and more particularly to a method for determining the first stage exit or impulse chamber temperature in high pressure steam turbines.
2. Description of the Prior Art
In the operation of multistage high pressure steam turbines, the rotor surface temperature closely follows the steam temperature while the interior rotor and bore responds more slowly, inducing thermal stresses. This results in low cycle thermal fatigue. Thus, the value of the steam temperature at the first stage exit is needed to permit control under widely varying load, as at startup, to minimize such stresses.
Typically, on starting, the turbine is brought up to speed, the generator synchronized, and a load of 5% applied with full-arc admission operation. As the load is increased, a transfer is made from full-arc admission to partial arc admission. This results in a step change in the first stage steam exit temperature. Such change may be 70.degree. F. for a minimum admission arc of 50%, and 100.degree. F. for a 25% minimum admission arc.
In this procedure the abrupt changes in steam temperature increase such thermal stresses.
Attempts have been made to minimize thermal stresses; for example, by a gradual transfer. In this approach, the valves corresponding to minimum admission are opened and the remaining valves closed. The rate of change of the first stage steam temperature is controlled by adjusting the rate of valve movement. This method therefore depends on an accurate measurement of the steam temperature. Commonly, a thermocouple is installed in the shell wall or other location at the impulse chamber for establishing the steam temperature.
However, the thermocouple measures the metal temperature rather than the steam temperature during changing conditions due to the inherent slow time of response. The metal temperature will be lower than the steam temperature, particularly during transients.
It is difficult to accurately measure the first stage steam temperature because of the high pressure, thicknesses of the metal shells, and slow response of the thermocouples. In the past, thermocouples for this purpose have been embedded in the shell or the base of the stationary blade of the next stage. However, the metal temperature is actually measured rather than the steam, The use of a well protruding into the steam path could give a more accurate measurement but presents a risk of breaking off and being carried into the flow path.
Turbines also experience temperature variations, which are of special concern at the first stage exit, during load changes because of the inherent mass flow-temperature characteristics of both the boiler and the turbine. Prompt detection of these temperature changes results in optimum rates of load change with improved turbine life.